mGluR7 Genetics and Alcohol: Intersection Yields Clues for Addiction

Development of addiction to alcohol or other substances can be attributed in part to exposure-dependent modifications at synaptic efficacy leading to an organism which functions at an altered homeostatic setpoint. Genetic factors may also influence setpoints and the stability of the homeostatic system of an organism. Quantitative genetic analysis of voluntary alcohol drinking, and mapping of the involved genes in the quasi-congenic Recombinant QTL Introgression strain system, identified Eac2 as a Quantitative Trait Locus (QTL) on mouse chromosome 6 which explained 18% of the variance with an effect size of 2.09 g/kg/day alcohol consumption, and Grm7 as a quantitative trait gene underlying Eac2 [Vadasz et al. in Neurochem Res 32:1099–1112, 100, Genomics 90:690–702, 102]. In earlier studies, the product of Grm7 mGluR7, a G protein-coupled receptor, has been implicated in stress systems [Mitsukawa et al. in Proc Natl Acad Sci USA 102:18712–18717, 63], anxiety-like behaviors [Cryan et al. in Eur J Neurosci 17:2409–2417, 14], memory [Holscher et al. in Learn Mem 12:450–455, 26], and psychiatric disorders (e.g., [Mick et al. in Am J Med Genet B Neuropsychiatr Genet 147B:1412–1418, 61; Ohtsuki et al. in Schizophr Res 101:9–16, 72; Pergadia et al. in Paper presented at the 38th Annual Meeting of the Behavior Genetics Association, Louisville, Kentucky, USA, 76]. Here, in experiments with mice, we show that (1) Grm7 knockout mice express increased alcohol consumption, (2) sub-congenic, and congenic mice carrying a Grm7 variant characterized by higher Grm7 mRNA drink less alcohol, and show a tendency for higher circadian dark phase motor activity in a wheel running paradigm, respectively, and (3) there are significant genetic differences in Grm7 mRNA abundance in the mouse brain between congenic and background mice identifying brain areas whose function is implicated in addiction related processes. We hypothesize that metabotropic glutamate receptors may function as regulators of homeostasis, and Grm7 (mGluR7) is involved in multiple processes (including stress, circadian activity, reward control, memory, etc.) which interact with substance use and the development of addiction. In conclusion, we suggest that mGluR7 is a significant new therapeutic target in addiction and related neurobehavioral disorders.     

Glutamatergic signaling in cellular transformation

The role of the glutamatergic system in cancer cell homeostasis has expanded exponentially over the last decade. Once thought to participate only in synaptic transmission and neuronal excitability, the presence of functional glutamate receptors has since been demonstrated in peripheral tissues. Most notable is the implication of glutamate receptors in the pathophysiology of various human malignancies. We previously described the oncogenic properties of metabotropic glutamate receptor 1 (Grm1), a G-protein-coupled receptor in melanoma development in vivo. TG-3, a transgenic mouse line, developed spontaneous melanoma with 100% penetrance in the absence of any known stimuli. Stable Grm1-mouse melanocytic clones display transformed phenotypes in vitro and were aggressively tumorigenic in vivo. Recent reports from other groups implicate two additional members of the metabotropic glutamate receptor family in melanomagenesis, overexpression of mGluR5 and activating mutations in GRM3. These findings highlight a previously underappreciated link between the glutamate signaling pathway and oncogenesis in melanoma biology, raising exciting possibilities in elucidating mechanisms in melanocyte transformation and exploring glutamate receptors as novel therapeutic targets. Here we further consider the potential mechanisms by which glutamate receptors can function as an oncogene leading to malignant transformation.     

Deletion of Metabotropic Glutamate Receptors 2 and 3 (mGlu2 & mGlu3) in Mice Disrupts Sleep and Wheel-Running Activity,and Increases the Sensitivity of the Circadian System to Light

Sleep and/or circadian rhythm disruption (SCRD) is seen in up to 80% of schizophrenia patients. The co-morbidity of schizophrenia and SCRD may in part stem from dysfunction in common brain mechanisms, which include the glutamate system, and in particular, the group II metabotropic glutamate receptors mGlu2 and mGlu3 (encoded by the genes Grm2 and Grm3). These receptors are relevant to the pathophysiology and potential treatment of schizophrenia, and have also been implicated in sleep and circadian function. In the present study, we characterised the sleep and circadian rhythms of Grm2/3 double knockout (Grm2/3^-/-) mice, to provide further evidence for the involvement of group II metabotropic glutamate receptors in the regulation of sleep and circadian rhythms. We report several novel findings. Firstly, Grm2/3^-/- mice demonstrated a decrease in immobility-determined sleep time and an increase in immobility-determined sleep fragmentation. Secondly, Grm2/3^-/- mice showed heightened sensitivity to the circadian effects of light, manifested as increased period lengthening in constant light, and greater phase delays in response to nocturnal light pulses. Greater light-induced phase delays were also exhibited by wildtype C57Bl/6J mice following administration of the mGlu2/3 negative allosteric modulator RO4432717. These results confirm the involvement of group II metabotropic glutamate receptors in photic entrainment and sleep regulation pathways. Finally, the diurnal wheel-running rhythms of Grm2/3^-/- mice were perturbed under a standard light/dark cycle, but their diurnal rest-activity rhythms were unaltered in cages lacking running wheels, as determined with passive infrared motion detectors. Hence, when assessing the diurnal rest-activity rhythms of mice, the choice of assay can have a major bearing on the results obtained.     

HPA-axis activity in alcoholism: examples for a gene-environment interaction.

Genetic and environmental influences are both known to be causal factors in the development and maintenance of substance abuse disorders. This review aims to focus on the contributions of genetic and environmental research to the understanding of alcoholism and how gene-environment interactions result in a variety of addiction phenotypes. Gene-environment interactions have been reviewed by focusing on one of the most relevant environmental risk factors for alcoholism, stress. This is examined in more detail by reviewing the functioning of the hypothalamic-pituitary-adrenal (HPA) axis and its genetic and molecular components in this disorder. Recent evidence from animal and human studies have shown that the effects of stress on alcohol drinking are mediated by core HPA axis genes and are associated with genetic variations in those genes. The findings of the studies discussed here suggest that the collaborations of neuroscience, psychobiology and molecular genetics provide a promising framework to elucidate the exact mechanisms of gene-environment interactions as seen to convene upon the HPA axis and effect phenotypes of addiction.     

Homer proteins regulate sensitivity to cocaine

Drug addiction involves complex interactions between pharmacology and learning in genetically susceptible individuals. Members of the Homer gene family are regulated by acute and chronic cocaine administration. Here, we report that deletion of Homer1 or Homer2 in mice caused the same increase in sensitivity to cocaine-induced locomotion, conditioned reward, and augmented extracellular glutamate in nucleus accumbens as that elicited by withdrawal from repeated cocaine administration. Moreover, adeno-associated virus-mediated restoration of Homer2 in the accumbens of Homer2 KO mice reversed the cocaine-sensitized phenotype. Further analysis of Homer2 KO mice revealed extensive additional behavioral and neurochemical similarities to cocaine-sensitized animals, including accelerated acquisition of cocaine self-administration and altered regulation of glutamate by metabotropic glutamate receptors and cystine/glutamate exchange. These data show that Homer deletion mimics the behavioral and neurochemical phenotype produced by repeated cocaine administration and implicate Homer in regulating addiction to cocaine.     

Glutamate and Brain Glutaminases in Drug Addiction

Glutamate is the principal excitatory neurotransmitter in the central nervous system and its actions are related to the behavioral effects of psychostimulant drugs. In the last two decades, basic neuroscience research and preclinical studies with animal models are suggesting a critical role for glutamate transmission in drug reward, reinforcement, and relapse. Although most of the interest has been centered in post-synaptic glutamate receptors, the presynaptic synthesis of glutamate through brain glutaminases may also contribute to imbalances in glutamate homeostasis, a key feature of the glutamatergic hypothesis of addiction. Glutaminases are the main glutamate-producing enzymes in brain and dysregulation of their function have been associated with neurodegenerative diseases and neurological disorders; however, the possible implication of these enzymes in drug addiction remains largely unknown. This mini-review focuses on brain glutaminase isozymes and their alterations by in vivo exposure to drugs of abuse, which are discussed in the context of the glutamate homeostasis theory of addiction. Recent findings from mouse models have shown that drugs induce changes in the expression profiles of key glutamatergic transmission genes, although the molecular mechanisms that regulate drug-induced neuronal sensitization and behavioral plasticity are not clear.     

谷氨酸受体与药物成瘾的相关研究

谷氨酸是中枢神经系统中最重要的兴奋性神经递质,其受体分为离子型和代谢型,受体激活后通过对Na+、K+、Ca2+等阳离子调节或通过与G蛋白偶联,从而激活一系列信号转导途径,参与记忆形成。药物成瘾是一种慢性、复发性脑疾病,以强迫性药物寻求以及丧失对药物使用控制能力为主要特征。研究表明谷氨酸受体与药物成瘾的发生发展有关,就谷氨酸受体在药物成瘾中作用的研究做一综述。     

Disruption of GRM1‐mediated signalling using riluzole results in DNA damage in melanoma cells

Gain of function of the neuronal receptor, metabotropic glutamate receptor 1 (Grm1), was sufficient to induce melanocytic transformation in vitro and spontaneous melanoma development in vivo when ectopically expressed in melanocytes. The human form of this receptor, GRM1, has been shown to be ectopically expressed in a subset of human melanomas but not benign nevi or normal melanocytes, suggesting that misregulation of GRM1 is involved in the pathogenesis of certain human melanomas. Sustained stimulation of Grm1 by the ligand, glutamate, is required for the maintenance of transformed phenotypes in vitro and tumorigenicity in vivo. In this study, we investigate the mechanism of an inhibitor of glutamate release, riluzole, on human melanoma cells that express metabotropic glutamate receptor 1 (GRM1). Various in vitro assays conducted show that inhibition of glutamate release in several human melanoma cell lines resulted in an increase of oxidative stress and DNA damage response markers.     

Interaction between ephrins/Eph receptors and excitatory amino acid receptors: possible relevance in the regulation of synaptic plasticity and in the pathophysiology of neuronal degeneration

There is increasing evidence that Eph receptors and their transmembrane ligands, named ephrins, interact with glutamate receptors in both developing and adult neurons. EphB receptors interact with proteins that regulate the membrane trafficking of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor subunits, and both ephrins and EphB receptors have been found to co-localize with N-methyl-d-aspartate (NMDA) receptors and to positively modulate NMDA receptor function. Moreover, pharmacologic activation of ephrin-Bs amplifies group-I metabotropic glutamate receptor signaling through mechanisms that involve NMDA receptors. The interaction with ionotropic or metabotropic glutamate receptors provides a substrate for the emerging role of ephrins and Eph receptors in the regulation of activity-dependent forms of synaptic plasticity, such as long-term potentiation and long-term depression, which are established electrophysiologic models of associative learning. In addition, these interactions explain the involvement of ephrins/Eph receptors in the regulation of pain threshold and epileptogenesis, as well as their potential implication in processes of neuronal degeneration. This may stimulate the search for new drugs that might modulate excitatory synaptic transmission by interacting with the ephrin/Eph receptor system.     

Synthesis and pharmacological characterisation of 2,4-dicarboxy-pyrroles as selective non-competitive mGluR1 antagonists

Metabotropic glutamate receptors (mGluRs) are an unusual family of G-protein coupled receptor (GPCR), and are characterised by a large extracellular N-terminal domain that contains the glutamate binding site. We have identified a new class of non-competitive metabotropic glutamate receptor 1 (mGluR1) antagonists, 2,4-dicarboxy-pyrroles which are endowed with nanomolar potency. They interact within the 7 transmembrane (7TM) domain of the receptor and show antinociceptive properties when tested in a number of different animal models.     

Accumbens Homer2‐mediated signaling: a factor contributing to mouse strain differences in alcohol drinking?

Alcohol‐induced increases in nucleus accumbens glutamate actively regulate alcohol consumption, and the alcohol responsiveness of corticoaccumbens glutamate systems relates to genetic variance in alcohol reward. Here, we extend earlier data for inbred mouse strain differences in accumbens glutamate by examining for differences in basal and alcohol‐induced changes in the striatal expression of glutamate‐related signaling molecules between inbred C57BL/6J and DBA2/J mice. Repeated alcohol treatment (8 × 2 g/kg) increased the expression of Group1 metabotropic glutamate receptors, the NR2a/b subunits of the N‐methyl‐d ‐aspartate receptor, Homer2a/b, as well as the activated forms of protein kinase C (PKC) epsilon and phosphoinositol‐3‐kinase within ventral, but not dorsal, striatum. Regardless of prior alcohol experience, C57BL/6J mice exhibited higher accumbens levels of mGluR1/5, Homer2a/b, NR2a and activated kinases vs. DBA2/J mice, whereas an alcohol‐induced rise in dorsal striatum mGluR1/5 expression was observed only in C57BL/6J mice. We next employed virus‐mediated gene transfer approaches to ascertain the functional relevance of the observed strain difference in accumbens Homer2 expression for B6/D2 differences in alcohol‐induced glutamate sensitization, as well as alcohol preference/intake. Manipulating nucleus accumbens shell Homer2b expression actively regulated these measures in C57BL/6J mice, whereas DBA2/J mice were relatively insensitive to the neurochemical and behavioral effects of virus‐mediated changes in Homer2 expression. These data support the over‐arching hypothesis that augmented accumbens Homer2‐mediated glutamate signaling may be an endophenotype related to genetic variance in alcohol consumption. If relevant to humans, such data pose polymorphisms affecting glutamate receptor/Homer2 signaling in the etiology of alcoholism.     

Alcohol and genetics: new animal models.

In recent years, it has become increasingly evident that there is a genetic component to alcoholism. Attempts to isolate alcoholism genes have met with modest success, in part because alcoholism is a multigenic trait. Recently, experimental animal models and novel genetic manipulations have provided several clues as to the specific genes involved in alcoholism, and extensive research has identified many genes that might influence responses to alcohol. Although not all of these might be proven to influence drug sensitivity, research has provided evidence for the involvement of a few genes. Ultimately, findings from animal models that investigate the function of specific genes could aid the development of pharmacotherapies to treat alcohol dependence.     

谷氨酸循环障碍与抑郁症

抑郁症是当今社会较多发的严重影响人类生活质量的疾病,迄今为止,抑郁症病因与发病机制还不完全明确,在过去的很多年,单胺类神经递质假说一直处在相对重要的位置,然而,近年越来越多的临床证据表明,单胺类递质假说并不能完全解释抑郁症的发病机制与其所起到的治疗作用,是否还有另一种机制参与其中呢？近期大量实验研究表明谷氨酸能系统在抑郁症的发病及治疗中扮演了重要的角色,本文就谷氨酸能系统在抑郁症的发病及治疗中发挥的疗效作用综述如下。     

The role of glutamate receptors in antipsychotic drug action

Summary. It has recently been postulated that disturbances in glutamatergic neurotransmission may contribute to the pathophysiology of schizophrenia. Therefore the aim of the present study was to evaluate the role of glutamate NMDA and group II metabotropic receptors in the antipsychotic drug action. To this aim the influence of some well-known neuroleptics on cortical NMDA receptors was examined. Furthermore, their behavioral effects were compared with those of the novel agonist of group II glutamate metabotropic receptors, LY 354740, in some animal models of schizophrenic deficits. We found that long-term administration of the typical neuroleptic haloperidol and the atypical one clozapine increased the number of NMDA receptors labelled with [³H]CGP 39653 in different cortical areas. Long-, but not short-term, treatment with haloperidol and raclopride diminished the deficit of prepulse inhibition produced by phencyclidine, which is a model of sensorimotor gating deficit in schizophrenia. In contrast, neither short- nor long-term treatment with clozapine influenced the phencyclidine effect in that model. Acute treatment with LY 354740 reversed neither (1) the deficit of prepulse inhibition produced by phencyclidine or apomorphine, nor (2) the impairment in a delayed alternation task induced by MK-801, which is commonly used to model the frontal lobe deficits associated with schizophrenia. The present study suggests that an increase in the density of cortical NMDA receptors may be important to a longterm neuroleptic therapy. Conversely, the results do not support the role of group II metabotropic glutamate receptors in the antipsychotic drug action. Received August 31, 1999 Accepted September 20, 1999     

Meta-analysis for microarray studies of the genetics of complex traits

In comparison to other complex disease traits, alcoholism and alcohol abuse are influenced by the combined effects of many genes that alter susceptibility, phenotypic expression and associated morbidity, respectively. Many genetic studies, in both animal models and humans, have identified genetic intervals containing genes that influence alcoholism or behavioral responses to ethanol. Concurrently, a growing number of microarray studies have identified gene expression differences related to ethanol drinking or other ethanol behaviors. However, concerns about the statistical power of these experiments, combined with the complexity of the underlying phenotypes, have greatly hampered the identification of candidate genes underlying ethanol behaviors. Meta-analysis approaches using recent compilations of large datasets of microarray, behavioral and genetic data promise improved statistical power for detecting the genes or gene networks affecting ethanol behaviors and other complex traits.     

Neuropeptides: implications for alcoholism

The role of neuromodulatory peptides in the aetiology of alcoholism has been relatively under-explored; however, the development of selective ligands for neuropeptide receptors, the characterization and cloning of receptors, and the development of transgenic mouse models have greatly facilitated this analysis. The present review considers the most recent preclinical evidence obtained from animal models for the role of two of the opioid peptides, namely b-endorphin and enkephalin; corticotropin-releasing factor (CRF), urocortin I and neuropeptide Y (NPY) in deleterious and excessive alcohol consumption, focussing on specific brain regions, in particular the central nucleus of the amygdala, that appear to be implicated in the pathophysiology of alcoholism. The review also outlines potential directions for further research to clarify neuropeptide involvement in neuromodulation within discrete brain nuclei pertinent to behavioural patterns.     

Phenotypic profiling of mGlu7 knockout mice reveals new implications for neurodevelopmental disorders

Neurodevelopmental disorders are characterized by deficits in communication, cognition, attention, social behavior and/or motor control. Previous studies have pointed to the involvement of genes that regulate synaptic structure and function in the pathogenesis of these disorders. One such gene, GRM7, encodes the metabotropic glutamate receptor 7 (mGlu₇), a G protein‐coupled receptor that regulates presynaptic neurotransmitter release. Mutations and polymorphisms in GRM7 have been associated with neurodevelopmental disorders in clinical populations; however, limited preclinical studies have evaluated mGlu₇ in the context of this specific disease class. Here, we show that the absence of mGlu₇ in mice is sufficient to alter phenotypes within the domains of social behavior, associative learning, motor function, epilepsy and sleep. Moreover, Grm7 knockout mice exhibit an attenuated response to amphetamine. These findings provide rationale for further investigation of mGlu₇ as a potential therapeutic target for neurodevelopmental disorders such as idiopathic autism, attention deficit hyperactivity disorder and Rett syndrome.